1. Field of the Invention
The present invention relates to a lead frame, and particularly to a lead frame for face-down bonding which is applied to bond a semiconductor chip having bumps to leads by ultrasonic wave with the semiconductor chip placed face down.
2. Description of the Related Art
A TAB (Tape Automated Bonding) technique has been hitherto known as a method of connecting bumps formed on electrodes of a semiconductor chip to leads.
FIG. 1 is a plan view showing an integrated circuit device which is connected to inner leads by using the TAB technique (Japanese Patent Application Laid-open No. Hei-4-109640).
As shown in FIG. 1, plural bumps 12 are formed at an electrode portion on the upper surface of a semiconductor chip 11. A tape carrier 15 comprises an insulating film with a device hole 16 having a larger size than the semiconductor chip 11, and plural TAB leads 13 which are provided so as to extend from the surface of the insulating film into the device hole. The semiconductor chip 11 is disposed at the center of the device hole 16, and each bump 12 which is provided at each electrode of the semiconductor chip 11 is individually bonded to the tip of each lead 13.
As shown in FIGS. 2A and 2B, a recess portion 14 is formed at the tip of each lead 13. The two side surfaces of the recess portion 14 positioned at both ends in the width direction of the lead 13 are slanted, and the bump 12 and the lead 13 are bonded to each other so that the slanted surfaces of the recess portion 14 are brought into contact with two confronting edge portions of the bump 12. As not shown, in the above conventional technique, the bonding of the lead 13 and the bump 12 has been performed by pressing down a heated bonding tool from the upper side of the lead 13 to heat the contact portion between the lead 13 and the bump 12.
A bonding technique based on ultrasonic bonding as shown in FIG. 3 has been known as another conventional technique (Japanese Patent Application Laid-open No. Hei-1-244630). According to this technique, a semiconductor chip 23 having bumps 24 is held by a collet 25 with the semiconductor chip 23 placed face down, the bumps 24 are brought into contact with the surface of a conductive pattern 22 on an insulating substrate 21 and then ultrasonic wave is applied to the collet 25 to bond the bumps 24 to the conductive pattern 23.
This ultrasonic bonding method has an advantage that it is unnecessary to set the press force and the degree of heat to large values and thus the damage to be imposed on the semiconductor chip is relatively small.
It has been a recent tendency in the mobile communication market that portable information equipment is designed in a small and light structure, and this tendency has increasingly required that a semiconductor device or semiconductor chip serving as a part of the portable information equipment is designed to be small in area and height when it is packaged. Accordingly, in order to reduce the outer dimension of the semiconductor device, a technique to which the TAB technique or the face-down bonding technique based on ultrasonic wave is applied have been required.
However, the conventional techniques as described above have the following problems.
A first problem resides in that when the semiconductor device is bonded to the lead of the conventional lead frame used in the TAB technique by the ultrasonic bonding, the bonding strength of the leads extending in the perpendicular direction to the application direction of ultrasonic wave is weaker than the bonding strength of the leads extending in the direction parallel to the application direction of ultrasonic wave.
The reason is as follows. As shown in FIGS. 4A and 4B, each lead 13 extending in the direction perpendicular to the ultrasonic wave application direction has two slant surfaces at both ends in the width direction of the lead, and these slant surfaces disturb the motion of the bump 12 due to the ultrasonic wave to make it hard to weld the contact surface of the bump with the lead. Further, the portions of the bump which are brought into contact with the two slant surfaces of the lead are repeatedly separated from and pressed to the slant surfaces of the lead in accordance with the application of the ultrasonic wave, resulting in unstableness of the bonding between the bump and the lead.
Further, a second problem resides in that there occurs dispersion in positioning between the bump and the lead extending in the direction perpendicular to the ultrasonic wave applying direction.
The reason is as follows. When the bump is moved at the tip of the lead in the lateral direction of the lead, that is, in the ultrasonic wave applying direction, the bump abuts the two slant surfaces of the lead in the lead width direction, and thus the tip of the lead is also swung in the width direction thereof.
Accordingly, in the case of the bonding of the bump and the lead by ultrasonic wave, the direction of the ultrasonic wave must be taken into consideration every lead to apply the ultrasonic wave in the direction of the lead, resulting in lower productivity.